Vancomycin intermediate-susceptible

MSSA, methicilUn-sensitive S. aureus MRSA, methiciUin-resistant S. aureus StrR, streptomycin resistant VISA, vancomycin intermediate susceptibility. 

Resistance to vancomycin in enterococci is due to modification of the D-Ala-D-Ala binding site of the peptidoglycan building block in which the terminal D-Ala is replaced by D-lactate. This results in the loss of a critical hydrogen bond that facilitates high-affinity binding of vancomycin to its target and loss of activity. This mechanism is also present in vancomycin-resistant S aureus strains (MIC s32. ug/mL), which have acquired the enterococcal resistance determinants. The mechanism for reduced vancomycin susceptibility of vancomycin-intermediate strains (MICs = 8-16 g/mL) is not known. 

S. aureus arul coagulase-negative staphylococci may express reduced or intermediate susceptibility to vancomycin or high-level resistance. Intermediate resistance is associated with (and may be preceded by) a heterogeneous phenotype in which a small proportion of cells within the population ( 1 in 10 ) will grow in the presence of vancomycin concentrations above 4 pg/mL. 

Infections caused by intermediate strains have failed to resporul to vancomycin. Prior treatment courses and low vancomycin levels may predispose patients to infection with such vancomycin-intermediate strains, which typically are resistant to methicillin and multiple other antibiotics. Their emergence is a major concern because vancomycin has been the only antibiotic to which staphylococci were reliably susceptible. 

The treatment of choice until susceptibility of the organism is known as the combination of vancomycin plus ceftriaxone. Penicillin may be used for drug-susceptible isolates with minimum inhibitory concentrations of 0.06 mcg/mL or less, but for intermediate isolates ceftriaxone is used, and for highly drug-resistant isolates a combination of ceftriaxone and vancomycin should be used. A high percent of S. pneumoniae is either intermediately or highly resistant to penicillin. 

Vancomycin/glycopeptide intermediately resistant and methicillin-resistant S. aureus have been described in Japan, Europe, the Far East, and the USA. Some vancomycin-susceptible strains of S. aureus contain subpopulations with intermediate resistance to vancomycin (heterogeneous strains), and these may escape laboratory detection (93,94,97-99). 

Organism exhibits intermediate resistance to vancocyn (vancomycin) concentration (disc) 5.0 yg. Organism is susceptible to vancocyn (vancomycin) concentration (disc) 7.5 yg. 

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